drm/i915: Refactor gen2 to gen4 vblank interrupt handling

The indentation is getting way too deep. Pull the vblank interupt
handling out to separate functions.

v2: Keep flip_mask handling in the main irq handler and
    flatten {i8xx,i915}_handle_vblank() even further.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>